The present invention concerns elevated temperature, thermal treatments of metallic or cermet materials and work parts in furnaces or reactors using reactive atmospheres. The atmospheres and treatments in the scope of invention include carburizing, nitriding, carbonitriding, nitrocarburizing, boronizing, bright annealing or oxide reduction, reducing atmospheres for brazing, soldering and sintering, carbon potential atmospheres for neutral heat treating of phase transformation alloys, solutionizing, aging, spheroidizing, hardening, stress relieving, normalizing, inert annealing, and the like. The components of said atmospheres may include nitrogen (N2), hydrogen (H2), hydrocarbon gases (HC) such as methane (CH4), acetylene (C2H2), ethylene (C2H4), propane (C3H8) and many heavier molecular weight hydrocarbons, ammonia (NH3), evaporated alcohols such as methanol (CH3OH) or ethanol (C2H5OH), carbon monoxide (CO), carbon dioxide (CO2), water vapor (H2O), and noble gases such as argon (Ar) and helium (He). Additional components of the atmosphere may include reaction byproducts and gases evolving from the furnace load or walls and/or heating components as well as the gases leaking into the furnace from outside, e.g. air. Atmosphere gases may be introduced into the furnace as blends, premixed up-stream of the furnace in the gas flow control system, or can mix inside the furnace chamber. The other options for atmosphere gas supply may include streams produced by endothermic and exothermic generators, e.g. the endothermic blend of 20% CO, 40% H2 and 40% N2 (unless otherwise stated, all percentages identified in this application should be understood to be on a volume basis) made by reforming CH4 with air, dissociated NH3, or injection and evaporation of liquids, e.g. CH3OH. There are three common process control problems when using atmospheres in elevated temperature thermal treatments: (1) slow kinetic reactions or stability of gas injected, (2) surface condition of material of work part loaded to furnace, and (3) environmental air leakage. For example, CH4 injected into the furnace for carburizing may slowly react with and dispose of undesired oxygen (O2) or CO2 and/or may shift the thermodynamic potential of furnace atmosphere, and/or may dissociate and react only marginally, unless furnace temperature is very high; however, the high temperature poses a risk of damaging the loaded metallic material or work part. Similar situations take place to a larger or smaller degree with H2, NH3, and the other HCs. Also, the material or work surface loaded into the furnace may be covered with a thick film of oxide, rust, or water-based oily residues, and the reactivity of the original atmosphere may turn out to be insufficient for this film removal within the desired treatment time and temperature range. Finally, furnace air leaks and the other O2-containing sources of contamination may require additions of reducing and, sometimes, carburizing gases to the atmosphere, even if the most desired atmosphere would be an inert environment to parts for specific thermal transformation processes, that is, one without reducing and/or carburizing gases. Such in-situ geftering techniques are limited by many process considerations. For example, the amount of H2 added to N2 atmosphere for reflowing solders on printed circuit boards has to be kept below 5% for safety, i.e. elimination of explosion risk in open or semi-open reflowing ovens, and the temperature has to be kept low, typically below 250° C., to prevent board and component damage. With these low temperatures and concentrations, the gettering and oxide removal effect of H2 is marginal due to slow reduction kinetics. Similar limitations can be found in carburizing of steel parts with natural gas, in the absence of endothermic, CO-containing atmospheres. For example, CH4 dissociates thermally and reacts with a steel surface at rapid, industrially attractive rates only above 1000° C., but many of the steels treated reveal an undesired grain growth effect at such high temperatures.
A number of ways have been used to deal with the problems described. Vacuum furnaces are used for thermal treatments to avoid environmental air leakage and evaporate impurity condensates from materials or parts loaded. Unfortunately, all vacuum furnace systems are expensive from the capital and operating standpoint. Moreover, the use of vacuum furnace doesn't solve the problem of gas stability. Thus, in the case of carburizing, more expensive and less stable hydrocarbons (HCs) have to be used, e.g. C2H2, and the use of the lowest cost CH4 is very limited. Ion plasma vacuum furnaces have been developed to cope with the problem of gas stability and the surface films initially covering loaded work parts, but the cost of those systems, issues with processing complex part geometries and the difficulty of controlling the temperature, limit the use of ion plasma systems. The additional complication with these and similar electric discharge methods, e.g. corona, direct arc or plasma transferred arc, is the requirement of making the work part (to be treated) one of the electrodes closing the discharge circuit. The furnace or reaction vessel becomes more complex and, in the case of intricate or electronic work parts, the current may damage the work parts. Non-transferred arc thermal plasma systems capable of operating at atmospheric pressures have been explored as gas-activating injectors that do not need to close the electric circuit via the work part. Nevertheless, the high temperature and current required in these systems shortens the life of electrodes to 100 hours or less and results in furnace contamination. The newest generation of microwave furnace systems eliminates the need for vacuum or low-pressure chamber and frequent electrode changes while activating the surface of loaded materials or work parts. Nevertheless, industrial-scale microwave furnace systems are complex, expensive and not as flexible in accepting diverse metallic materials and geometries of work parts as the traditional, thermally heated furnaces.
Drissen at al. (U.S. Pat. No. 5,717,186) proposed additional measures for controlling the direct current flowing through a workpiece in an ionic, vacuum heat treatment furnace. Law et al. (U.S. Pat. No. 5,059,757) devised a way of limiting sooting in the same type of furnace. Orita (U.S. Pat. No. 5,605,580) used a multi-step heat treatment procedure to minimize a non-uniform edge-carburizing effect, much more acute in the vacuum plasma systems than in the conventional gas carburizing. Georges (U.S. Pat. No. 5,989,363) demonstrated the need for radiation screens in post-discharge, vacuum plasma nitriding. Giacobbe (European Patent No. 0324294A1) described workpiece surface and through hardening using a water-cooled thermal plasma torch. He and Paganessi (PCT Publication No. WO2005/009932A1) also used a high-power (50-500 kW) plasma reactor to generate treatment gases that were, subsequently, injected into a vacuum furnace. Czernichowski (U.S. Pat. No. 6,007,742) disclosed a series of experimental, normal atmosphere pressure, “GlidArc” plasma methods for reforming natural gas and other hydrocarbons. Hundreds of research papers exist concerning the use of more or less elaborate, typically low-pressure, laboratory-scale plasma systems for metal treatment in the heat treating applications as well as modification of gas stream composition. Nevertheless, a large portion of the metal heat treating industry and thermal process operators continue to look for an improved, reactive atmosphere system could enhance kinetics of reactions in gas phase and at work surfaces while, simultaneously, may be retrofitted to the existing, normal or reduced pressure furnaces or reactors, and would not necessitate high capital or operating and maintenance expenses.